i just bought a 98 gt auto with about 85,000. it ran great when i took it for a drive and on the way home, the next day when i went to start it it was horrible. it turned over fine, but would barely idle for about 30 seconds, then it idled fine...? i filled it up with gas on the way, could it be bad gas? the only mods he said it has is highflow cats. when your cruisin down the highway, after about 30 seconds it drags the RPM's down to 1400 in a hurry, is it suppose to do this, it runs fine once its runnin. 
Maybe you should scan for codes. I'd also ask the prior owner what's up with that problem.
